Today when I printed my label from the electronis postage on PBS I forgot to change the ship date to today (I was originally going to mail out Friday, but Decided to do it today). Can I just cross it out? Does it matter?

Don't try to cross it out, the electronic postage should not be altered. As long as the date is in the future instead of in the past it should be ok. I've only seen one member post that it was a problem to mail ahead of time. If the postage is for a past date (stale postage) they will often not accept it - makes them look slow; but a date in the future shouldn't matter.